1. Field of the Invention
The present invention relates to an optical scanning unit and an image forming apparatus using the optical scanning unit, and more specifically, an optical scanning unit to optically write an image and an image forming apparatus using the optical scanning unit.
2. Description of the Background
Typically, an image forming apparatus used as a printer, facsimile machine, copier, and multi-functional device thereof has an optical scanning unit to write a latent image on an image carrier or photoconductor. Such an optical scanning unit is constructed to deflect a light flux or beam emitted from a light source by a rotary deflector and thus expose and scan the photoconductor with the light beam.
One conventional image forming apparatus forms a latent image on an image carrier by such an optical scanning unit, visualizes the latent image as a toner image by a developing unit, transfers and fixes the toner image on a recording material, such as a transfer paper sheet, by a transfer unit and a fixing unit, and discharges the recording material from the image forming apparatus.
Recently, for such an image forming apparatus, there has been a demand for compatibility with a wide variety of paper types including a postcard, release paper, and thin paper such as tracing paper, in addition to plain paper.
One difficulty faced in attempting to accommodate such different media is that, for example, fixing a toner image on a thick sheet may need a relatively large amount of heat compared to fixing the same image on an ordinary sheet of plain paper. Therefore, one type of conventional image forming apparatus reduces the processing speed of a photoconductor (hereinafter “linear velocity”) so as to reduce its printing speed. By so doing, this type of conventional image forming apparatus increases the heat amount per unit time to secure stable fixing performance, with the printing speed for such thick paper set slower than the printing speed for plain paper.
Further, in response to recent increasing needs for color printing, there have been proposed image forming apparatuses capable of forming full-color images by superimposing, one on top of the other, toner of four colors of black, magenta, cyan, and yellow, for example. Such a conventional full-color image forming apparatus needs a relatively large amount of heat when fixing toner of four colors compared to when fixing toner of the single black color, for example. Consequently, the conventional full-color image forming apparatus forms a full-color image at a reduced printing speed compared to when forming a black-and-white image.
As described above, in response to various demands, certain image forming apparatuses are capable of operating a photoconductor at a plurality of linear velocities and switching the linear velocities of the photoconductor depending on operation modes defined by paper type, monochrome or color printing, and the like.
In this regard, the action of such an optical scanning unit to the linear velocity of a photoconductor is described below.
For example, where the linear velocity of the photoconductor is “V” mm/s, a number of reflective faces of a rotary deflector is “M”, a number of light beams directed onto a surface of the photoconductor is “N”, and a pixel density is “ρ” dpi (dot per inch), the rotation speed “Rm” of the rotary deflector in the optical scanning unit is expressed by the following equation:Rm=(60×ρ×V)/(25.4×M×N).
As indicated in the above equation, normally, as the linear velocity V of the photoconductor increases, the rotation speed Rm of the rotary deflector also increases. By contrast, as the linear velocity V decreases, the rotation speed Rm also decreases.
A DC (direct current) brushless motor is generally used as a motor in the rotary deflector. For such a motor, the optimal range of its rotation speed is determined to a certain degree by the optimal range of an input clock to the motor, bearing type or structure, or the like.
However, when forming an image at a reduced linear velocity of the photoconductor in thick-sheet printing or full-color printing, the motor may be operated at a rotation speed significantly lower than the optimal range, thereby worsening low-frequency jitter or uneven rotation of the rotary deflector. Consequently, image failures such as image fluctuation may be generated.
Accordingly, one proposed optical scanning unit is capable of simultaneously scanning a plurality of light beams onto a surface of a photoconductor drum, and also reduces the number of light beams directed onto the surface of the photoconductor if the linear velocity V of the photoconductor is lower than a certain value. As implied in the equation above, such a reduction in the number of light beams can offset a reduction in the rotation speed Rm of the rotary deflector that may be generated by a reduction in the linear velocity V of the photoconductor in thick-sheet printing or full-color printing.
As a result, the motor of the rotary deflector may still operate within the optimal range of the rotation speed even when forming an image at such a reduced linear velocity V of the photoconductor, thereby suppressing low-frequency jitter or uneven rotation of the rotary deflector.
In such an optical scanning unit, a scan speed Vimg of a scan line is proportional to the rotation speed Rm of the rotary deflector, and therefore proportionally decreases with a reduction of the rotation speed Rm of the rotary deflector. When a light flux or beam from a light source in such an optical scanning unit scans the surface of the photoconductor, the photoconductor is rotating at a linear velocity V. As a result, a scan line by the light flux may be inclined relative to an ideal or reference scan line depending on relative speed between the scan speed Vimg of the scan line and the linear velocity V of the photoconductor.
For example, where “L” represents a length of one scan line scanned across the photoconductor, the time “t” in which the scan line is scanned across the photoconductor is t=L/Vimg. Further, an inclination amount α of the scan line relative to the reference scan line in a sub-scan direction is expressed by the following equation:α=t×V=(V/Vimg)×L. 
The optical scanning unit is constructed so that a scan line is not inclined relative to such a reference scan line when printing an ordinary sheet of plain paper. Accordingly, if the rate of reduction of the linear velocity V is identical to the rate of reduction of the scan speed Vimg, the inclination amount α also remains identical, and thus the scan line is not inclined relative to the reference scan line in the sub-scan direction.
However, according to one conventional optical scanning unit, when a rate of reduction of the rotation speed Rm is relatively small compared to a rate of reduction of the linear velocity V, the rate of reduction of the scan speed Vimg, which is proportional to the rotation speed Rm, also decreases compared to the rate of reduction of the linear velocity Vimg. As a result, a ratio K of the linear velocity V to the scan speed Vimg (K=V/Vimg) may be changed. A change in the ratio K of the linear velocity V to the scan speed Vimg may result in an inclination in the scan line scanned across the photoconductor, thereby generating an inclined image.
Furthermore, even when the rotary deflector is controlled to operate at a designated rotation speed, with extended use over time the rotary deflector may begin to rotate at a rotation speed that deviates from the designated rotation speed, shifting the linear velocity V from a set value. In such a case, the ratio K of the linear velocity V to the scan speed Vimg may also be changed, thereby generating an inclination of the scan line and an inclined image.
Thus, there is still a need for an optical scanning unit capable of suppressing an inclination of a scan line when the ratio between the linear velocity of an image carrier and the scan speed of the scan line changes, and an image forming apparatus having such an optical scanning unit.